TACAGUA, Formation

MIDDLE or UPPER MESOZOIC

Distrito Federal, Venezuela

Author of name: G. Dengo, 1951.

Original reference: G. Dengo, 1951, p. 66.

Original description: ibid.

The name Tacagua formation was proposed by G. Dengo (1951, p. 66) for a unit, the rocks of which he had previously described (1947, p. 138). It transitionally overlies the Las Mercedes formation. Its type-locality is the Tacagua Valey, immediately to the north of its junction with the Quebrada Topo, where the lower part of the formation is interdigitåted with the upper part of the Las Mercedes calcareous schists.

The formation is described as a fine-grained, light green, and uniformly foliated schist, made-up largely of sericite and epidote. It is more competent than the Las Mercedes formation. Towards the lower part it is slightly calcareous; some white bands are mainly quartz and albite; quartz veins also occur but are fewer than in the Las Mercedes formation. Dengo (p. 67) points out the fact that the original material constituting the Tacagua formation was largely volcanic, probably a tuff or a sediment containing volcanic ashes. The formation was not observed towards the south of the type-area, called by Dengo (1951, fig. 5, front p. 68) "El Avila anticline region"; it does not outcrop in the "Cementerio syncline region" or in the "Sebastopol-Baruta anticline region".

The upper contact, apparently erosional, was not determined with precision by Dengo, since Tacagua "is the uppermost metamorphic formation observed in the Caracas region". "North of Quebrada Topo, the sericiticepidote schist is 150 to 200 m. thick but the top of the formation has been eroded" (Dengo, 1951, p. 66). Dengo (Table 3, front p. 70) includes the Tacagua formation in the upper part of the Caracas group. Smith (1952, p. 363) finds that the Las Mercedes is the uppermost formation of the Caracas "series" (group) observed in the Los TequesCúa area. It lies in fault contact under the Paracotos formation. Smith believes that the Paracotos formation, obviously of a lower metamorphic grade, unconformably overlies the Caracas group. He further states (p. 369) that part of the rocks considered by Dengo as Tacagua formation are "lithologically similar to the middle Paracotos". "An additional investigation might make a correlation between such rocks possible". The occurrence of two green schist units somewhat similar lithologically but different in age, is thus suggested: a) the facies overlying (according to Dengo, interdigitated with) the Las Mercedes schists and outcropping at the typelocality of the Tacagua formation, which pertains to the Caracas group; b) a similar facies located in the area surveyed by Dengo which, according to Smith, might belong to the middle Paracotos, that is, much younger in age since it is a part of a less metamorphosed sequence that unconformably overlies the Caracas group.

Another possibility would be the correlation, suggested by Smith, of the Tacagua formation with the Paracotos formation, or part of it. In that case, the stratigraphic relations between said units and the Caracas group should be carefully reviewed in order to decide once and for all whether the Tacagua lower contact is really transitional as it was suggested by Dengo, or if on the contrary, an unconformity exists. Such unconformity must exist if the Tacagua formation is equivalent to the middle Paracotos formation.

It is very important that future investigations make this point clear in order to get a clear idea of the relative chronology of the vulcanism observed in the eugeosyncline and its real position on the stratigraphic column. (See PARACOTOS FORMATION, ARRAYANES GROUP, TIARA VOLCANIC ROCKS, and SAN JUAN DE LOS MORROS FORMATION).

J. M. Sellier de Civrieux

PARACOTOS, Formation

UPPER CRETACEOUS

State of Miranda, Venezuela

Author of name: R. J. Smith.

Original reference: R. J. Smith, 1952, p. 363.

Original description: ibid.

The name Paracotos formation was introduced in the geological literature by R. J. Smith (1952, p. 363) to designate a group of phyllitic rocks, unconformably overlying the Caracas group. At the area of Los Teques, the contact is a fault contact. The best outcrops are found at the localities of Paracotos, Rio Tuy, El Paují, Quebrada Suapire, and south of Guayas. (ibid., geological chart of Los Teques-Cúa region). In the southern part of the area, the volcanic rocks of Tiara and Tertiary and Quaternary sediments overlie the Paracotos formation. This formation was intruded by peridotite, hornblende diorite and soda granite. The total thickness of the formation at the Los Teques area is of some 2,300 meters.

The Paracotos formation has been subdivided by Smith (1952, p. 363) into:

Lower Paracotos: The lowermost beds of the formation are constituted by phyllites composed of essential chlorite and quartz with accessory zircon, zoicite, garnet and tourmaline. Those beds show a defined lineation and intense folding. Limestone beds interbedded with phyllites and numerous quartz veins are noted. Above these phyllites is a zone of microconglomerates composed predominantly of chert with a few quartz grains, up to 2 mm. in diameter, in a green chloritic matrix; graphite is observed in some of these rocks. There are phyllites too fine in texture for precise optical determination. The X ray determination shows the high percentage of quartz content. It can be that this material represents metamorphosed tuffs. A thin limestone bed occurs near the upper contact with the green beds of the overlying section.

The thickness of this part of the section is estimated to be 900 meters.

Middle Paracotos: That section is quite distinctive in its outcrops of the lower Paracotos; it is constituted of resistant green beds. In most of the localities observed by Smith, it conformably overlies the phyllitic lower layers; in some localities the contact is a fault contact. In its freshest outcrops, the rock is massive, green, and it shows little schistosity; the greenish color is due to the high percentage of fine-grained epidote scattered in the rock; the remaining minerals are chlorite, quartz, zoicite and leucoxene. The greenstones of the middle Paracotos are among the most resistant rocks of the whole formation.

Some beds of chert can be found within the greenstones. Quartz veins are abundant, specially in the upper part.

The thickness of the middle Paracotos is about 1,200 meters.

Upper Paracotos: A series of phyllites, graywackes, conglomerates, and limestones constituting the upper part of the formation, conformably overlies the green beds of the middle Paracotos. Approximate thickness: 200 meters.

The paleontological determination carried out by H. H. Renz on a fossiliferous pebble collected by H. H. Hess among those slightly metamorphosed conglomerates at the road between El Valle and Charallave is considered of remarkable importance. The dark green limestone fragment contained foraminifera: Gümbelina sp. (common); Globigerinidae (common); Globotruncana sp. aff. apenninica Renz.

According to Renz, the fragment is Upper Cretaceous in age, possibly Cenomanian, equivalent to the lower part of the Querecual and La Luna formations.

On the other hand, J. M. S. de Civrieux (private report) identified in limestone lenses of the Paracotos phyllites, a microfauna confining radiolaria, Globigerina, Bulimina cf. prolixa, and Dentalina ? which suggests a problable Senonian age. The samples were collected by R. Laforest around the Km. 23 of the road Guatire-Caucagua. H. H. Hess (unpublished letter to de Civrieux, Feb. 16th, 1953) considers the identification as a "good check of the age of the Paracotos formation and of its correlation with the lower Guayuta group". Another fossiliferous sample, identified by de Civrieux, containing a similar microfauna (radiolaria, Gümbelina, Globigerina and Globotruncana) comes from a locality 2.5 Kms. from the bridge of Santa Teresa on the road from Santa Teresa to Guatopo. The rocks of such locality consists of thick layers of dark gray to black limestones, partly recrystallized, well bedded, and showing an almost vertical dip, which seem to pertain also to the Paracotos section.

Recently, H. H. Renz found paleontological evidences which demonstrated that part of the Paracotos is Maestrichtian. The fossiliferous samples studied by Renz come from a locality some 40 kms to the east of Villa de Cura.

Subsequent to their deposition, the rocks of the Paracotos formation were subjected to a low grade metamorphism. The development of chlorite chloritoid, sericite, and scarce biotite suggests that most of those rocks pertain, according to Smith, to the chlorite-biotite subfacies of the green schists facies of Turner' clasification.

S. E. Aguerrevere and G. Zuloaga (1937, p. 23) considered what Smith calls Paracotos formation as part of their Villa de Cura "serie" (see).

At the Caracas region, some of the rocks pertaining to the Tacagua formation described by Dengo (1949, p. 45) are lithologically similar to the middle Paracotos formation. Hess and Maxwell's Los Robles group (1949, p. 1860) at the Island of Margarita is also lithologically similar.

A. Bellizzia